1. Field of the Invention
The present invention relates to an image coding apparatus, and more particularly to an image coding apparatus which divides an image into blocks and codes it for each block.
2. Related Background Art
As one of coding methods for highly compressing an image signal while suppressing the deterioration of the image signal (high efficiency coding), a block coding method which divides an image into a plurality of blocks and codes it for each block has been known.
A representative method for coding an image signal in a block is a DCT (Discrete Cosine Transform) coding method.
In the DCT coding method, pixel data in a coding block are converted to DCT coefficient data representing a spatial frequency distribution by the DCT conversion and the DCT conversion coefficients are compressed in accordance with the fineness of the image and a visual characteristic.
The above high efficiency coding method enables the transmission at a low transmission rate but it includes a large affect by a transmission error.
Accordingly, it is necessary to countermeasure against the code error such as an error detection and correction code.
Particularly when the coded data is to be transmitted through an electromagnetic transformation system such as a magnetic recording or a BS communication system, the deterioration of the transmission quality is anticipated and hence the countermeasure for the code error is essential.
FIG. 1 shows a block diagram of an image transmission system by the high efficiency coding. An image signal to be transmitted is applied to an input terminal 10. An A/D converter 12 digitizes an analog signal from the input terminal 10 and a block forming circuit 14 divides the image data from the A/D converter 12 to coding blocks of 8.times.8 pixels or 4.times.4 pixels and outputs them in the order of coding blocks. In FIG. 2, one frame of image is divided into blocks of 4.times.4 pixels.
A high efficiency coding circuit 16 highly efficiently codes the image data of the coding blocks from the block forming circuit 14 for each coding block. Thus, the information quantity is compressed. An error correction coding circuit 18 codes with an error correction function the image data compressed by the high efficiency coding circuit 16. Namely, it calculates an error detection and correction parity and adds it to the compressed image data. The compressed image data which has been error correction coded by the error correction coding circuit 18 is outputted to a transmission line 20.
The transmission line 20 may be a wired or wireless communication medium such as an optical fiber, a BS or a microwave, or a recording and reproducing system such as a magnetic tape, a magnetic disk, an optical disk or a semiconductor memory (for example, a digital video tape recorder (VTR) or a digital audio tape recorder (DAT)). The transmission rate may be several tens K bits/sec to several tens M bits/sec although it depends on the information quantity of the original image, the compression rate and the transmission time.
In a receiving station, the data transmitted through the transmission line 20 is temporarily stored in a memory 22, and an error correction circuit 24 corrects an error in the transmitted data by the error correction parity of the transmission. The data stored in the memory 22 and having the error corrected is supplied to a high efficiency decoding circuit 26, which expands the compressed image data.
For an encoding block including a transmission error which has not been corrected by the error correction circuit 24, an interpolation circuit 28 interpolates by replacing it with the data of the encoding block at the same position in the previous frame (or field). The D/A converter 30 converts the output of the interpolation circuit 28 to an analog signal which is supplied to an output terminal 32.
In the image transmission system shown in FIG. 1, it frequently happens that the transmission error created in the transmission line 20 exceeds an error correction ability of the error correction code by the error correction coding circuit 18. As a result, the interpolation ability of the interpolation circuit 28 significantly affects to the quality of the reproduced image.
In a prior art system, the unit of processing of the interpolation circuit 28 is equal to the encoding block.
Namely, in the prior art interpolation method, the coding block having an incorrectable error is interpolated by using the coding block at the same image position in the immediately previous frame.
Accordingly, when the coding block is considerably large, the unit of processing of the interpolation is large and the effect of interpolation is reduced.
Further, since the interpolating block is spaced in time and space, it may be visually recognized.